1. Field of the Invention
The present invention relates to a color cathode ray tube set, and more particularly to a degaussing method and apparatus for a shadow mask type color cathode ray tube set.
2. Description of the Related Art
In general, a color cathode ray tube of a shadow mask type comprises a panel section including a substantially rectangular faceplate and a skirt extending from the peripheral portion of the faceplate; a funnel section connected to the panel; and a neck section continuous to the funnel. By these three sections, the interior of the cathode ray tube is hermetically sealed. An electron gun assembly for emitting electron beams is arranged inside the neck. A deflection yoke for generating a magnetic field is provided on the outer surface of the funnel and neck, and a degaussing coil for degaussing a magnetized member is provided on the outer surface of the funnel. A phosphor screen is formed on the inner surface of the faceplate of the panel. A substantially rectangular shadow mask is arranged inside the tube such that it faces the faceplate, with a predetermined distance maintained. The shadow mask is made of a thin metal plate having a large number of slit apertures. A mask frame, having a plurality of elastically deformable mask support members welded thereto, is provided around the shadow mask. A plurality of stud pins, which are engaged with the respective mask support members, are arranged on the inner surface of the panel. An internal magnetic shield is provided on that side of the mask frame which faces the neck, so as to prevent the electron beams emitted from the electron gun assembly from being affected by magnetic fields of earth magnetism et al.
In the color cathode ray tube having the above construction, the three electron beams emitted from the electron gun assembly are deflected both horizontally and vertically, due to the magnetic field generated by the deflection yoke, and are then converged toward the respective slit apertures of the shadow mask. After converging in the vicinity of the slit apertures, the electron beams land on the phosphor screen formed on the panel. The phosphor screen has three kinds of phosphor stripes arranged alternately. These phosphor stripes emit red, green and blue light rays, respectively, when the electron beams passing through the slit apertures of the shadow mask are incident thereon. That is, the slit apertures of the shadow mask serve to direct the three kinds of electron beams to the red-, green-, and blue-color producing phosphor stripes, respectively.
Normally, the shadow mask, mask frame, internal magnetic shield and other members of a color cathode ray tube set are formed of a magnetic material, such as low carbon steel. Therefore, if they become magnetized due to magnetic field of the earth magnetism et al, their remnant magnetism may shift the paths of electron beams. If the paths of electron beams are shifted, the electron beams do not land accurately on the phosphor screen, with the result that the color purity of the color cathode ray tube apparatus deteriorates. To avoid this deterioration, the magnetized members have to be demagnetized to erase the remnant magnetism therefrom.
Conventionally, the magnetized members of a color cathode ray tube set are degaussed by means of a degaussing coil wound around the outer wall of the funnel of the tube. Degaussing is normally carried out in the following three cases:
(1) The first degaussing is effected in the manufacturing process of a color cathode ray tube, so as to test the characteristics of a color cathode ray tube which has been manufactured.
(2) The second degaussing is effected in the manufacturing process of a TV set, so as to test the characteristics of a TV set into which the color cathode ray tube and other components have been incorporated.
(3) The third degaussing is effected every time the TV set is switched on.
In the test of the characteristics of the color cathode ray tube, the color purity is measured for the evaluation of the picture quality. This measurement is performed after remnant magnetism is erased by use of the degaussing method mentioned later.
The color cathode ray tube assembled in the TV set is demagnetized to erase the remnant magnetism for controlling the color purity when the TV set is adjusted.
A conventional degaussing method will be described. The electron beams emitted from an electron gun assembly are deflected by the vertical and horizontal deflection coil of the deflection yoke driven by a vertical and horizontal deflection current. To erase the remnant magnetism from magnetic members, a transient, attenuating A.C. current (i.e., an A.C. current which gradually attenuates with time) is supplied to the degaussing coil while or after the phosphor screen is scanned with the electron beams. The frequency of the A.C. current is the same as the commercial frequency, and the frequency of the vertical deflection current is also the same as the commercial frequency in most regions in the world. FIG. 1 shows how degaussing magnetic field 2 generated by the transient, attenuating A.C. current and deflection magnetic field 4 generated by the vertical deflection current change with time. As is shown in FIG. 1, degaussing magnetic field 2 and deflection magnetic field 4 have the same frequency. Degaussing magnetic field 2 is shown as being in phase with deflection magnetic field 4, but normally, degaussing magnetic field 2 is not generated in synchronism with deflection magnetic field 4. Therefore, when degaussing magnetic field 2 is applied for the degaussing of the color cathode ray tube, a phase shift is likely to occur between degaussing magnetic field 2 and deflection magnetic field 4. FIG. 2 shows magnetic flux density distribution 6 relating to the vertical deflection magnetic field generated when the deflection coil is supplied with a vertical deflection current. In FIG. 2, magnetic flux densities are plotted against the ordinate and distances measured from the neck are plotted against the abscissa. As is indicated by magnetic flux density distribution 6, the magnetic flux density is as high as 5 gausses even at position 8 where the end of the magnetic shield is located. Since, therefore, the degaussing magnetic field and the vertical deflection magnetic field are superimposed on each other in the location of the magnetic shield, a hysteresis loop of a magnetic member applied with both degaussing magnetic field 2 and deflection magnetic field 4 is not symmetric with reference to the origin, and the magnetic shield retains the magnetism arising from the above-mentioned phase shift, even after it is degaussed. FIG. 3 shows hysteresis loop 10 of the magnetic shield. As is shown, hysteresis loop 10 of a magnetic member applied with both degaussing magnetic field 2 and deflection magnetic field 4 is transformed or shifted from normal hysteresis loop 12, which is indicated by the broken lines with the reference numeral of "12". Although, in FIG. 3, hysteresis loop 12 is rotation-symmetric with reference to the origin, hysteresis loop 10 is not. FIG. 4 shows a detailed hysteresis curve obtained when a magnetic material is degaussed by applying a degaussing magnetic field thereto. FIG. 5 shows a degaussing magnetic field generated when a degaussing current flows through the degaussing coil, and also shows a vertical deflection magnetic field generated when a vertical deflection current flows through the deflection yoke. The magnetic flux density and magnetic field strength at time a in FIG. 5 are indicated at point a in FIG. 4, and those at time c in FIG. 5 are indicated at point c in FIG. 4. Likewise, times b and d-h in FIG. 5 correspond to points b and d-h, respectively.
Otherwise, the magnetic flux density and the magnetic field strength at time a in FIG. 5 are indicated at point a' in FIG. 4 when the deflection yoke does not generate a vertical deflection magnetic field. Those at time c in FIG. 5 are indicated at point c' in FIG. 4 when the yoke does not generate the magnetic field. Likewise, times e, g in FIG. 5 correspond to points e', g', and times b, d, f and h in FIG. 5 correspond to proximate points (not shown) of points b, d, f and h respectively when the yoke does not generate the magnetic field. As a result, the hysteresis curve in FIG. 4 is shifted from a hysteresis curve (not shown) in the case of the magnetic shield applied with only degaussing magnetic field. Shift distance between point c and point c' is shorter than shift distance between point a and point a'.
Therefore, when the magnetic shield is applied with both degaussing magnetic field 21 and vertical deflection magnetic field 22, its hysteresis curve 20 can be represented in the manner indicated in FIG. 4. Accordingly, magnetic field strength Hb at time b is greater than magnetic field strength Hd at time d since magnetic shield is applied with vertical deflection magnetic field 22. The decrease quantity .DELTA.Hd of magnetic field strength Hd is greater than the decrease quantity .DELTA.Hb of magnetic field strength Hb. Therefore, hysteresis curve 20 is formed asymmetrically shown in FIG. 4. Moreover, the asymmetrical hysteresis curve is shifted in one direction during this degaussing since vertical deflection magnetic field 22 at points a, b, c and d in FIG. 5 are equal to magnetic field 22 at points e, f, g and h respectively. As a result of this degaussing, the hysteresis curve converges at point r in FIG. 4, and the remnant magnetism at converging point r is Br. In short, the remnant magnetism does not decrease to 0. Since converging point r moves due to the phase difference between the degaussing magnetic field and the vertical deflection magnetic field, remnant magnetism Br varies accordingly.
FIG. 6 shows how the landing point of an electron beam is shifted from its initial landing point on the phosphor screen corner, wherein the initial landing point is obtained when the color cathode ray tube is degaussed at the first time by using the above-described degaussing method, and the other landing point is obtained when the color cathode ray tube is also degaussed at the other time. In FIG. 6, the ordinate represents the distance between the initial landing point and the other landing point, while the abscisa represents how many times the degaussing method has been used. As can be understood from FIG. 6, the maximum shift distance is 33 .mu.m and the average shift distance is 11 .mu.m. Since as noted above, the remnant magnetism varies in accordance with the phase difference between the degaussing magnetic field and the vertical deflection magnetic field, therefore the distance through which the landing point is shifted varies in accordance with the phase difference.
In an embodiment of U.S. Pat. No. 4,737,881, a resonance circuit is used for degaussing a magnetized member. The resonance circuit comprises capacitor which has several micro farads of capacitance C and coil which has several millihenries of inductance I. According to f=1/2.pi..sqroot.IC, the degaussing frequency f in the circuit is several ten kHz. The degaussing energy E is shown in E.varies.2.pi..sqroot.IC=1/f. The energy E is inversely proportional to the frequency. Therefore, the energy E is smaller as the degaussing frequency is higher. As a result, the magnetized member is not completely degaussed by the degaussing apparatus when degaussing frequency is very high.
Although a resonance circuit which oscillates under 100 Hz frequency is well known, such a circuit has to use a capacitor which has several farads of capacitance C and a coil which has several henries of inductance I. Therefore, the size of the circuit is larger than the size of a color cathode ray tube set. Moreover, the cost of the circuit is at least ten times greater than that of the set. Hence, it is impractical to use the circuit in the set.
If the set comprises the resonance circuit which oscillates at a frequency of several tens of kHz, the degaussing frequency is much higher than that of a vertical deflection magnetic field. Since the degaussing energy in the circuit is small and the magnetized member is affected by the field, the member is therefore not completely degaussed by the degaussing apparatus.